Background/Aims: This study was designed to investigate whether the serum IgA/C3 ratio can be a serologic marker of disease activity in children with severe Henoch-Schönlein nephritis (HSN). Methods: Twelve HSN patients who were treated with steroids and cyclosporine were examined. The levels of serum IgA and C3 were measured using an international reference preparation (IFCC/CRM470) and a renal biopsy was performed in all patients before and after therapy. After therapy, patients were divided into 3 groups: complete remission (n = 6, group I), mild urinary abnormalities (n = 3, group II), and active renal disease (n = 3, group III). Results: The serum IgA/C3 ratio decreased significantly in groups I and II after therapy (2.62 ± 0.82 vs. 2.02 ± 0.52, p = 0.02), whereas the ratio in group III increased, although it was not statistically significant (2.13 ± 0.93 vs. 4.67 ± 1.71, p = 0.25). A follow-up renal biopsy revealed that the activity index was reduced in groups I and II (7.0 ± 2.4 vs. 3.6 ± 1.6, p = 0.016), and not changed in group III (7.3 ± 2.1 vs. 9.3 ± 2.5, p = 0.25). The activity index at a follow-up renal biopsy correlated positively with the changes of the serum IgA/C3 ratio: posttherapy activity index = 1.20 × ΔIgA/C3 + 4.78 (r = 0.635, p = 0.027); where ΔIgA/C3 is posttherapy IgA/C3 – pretherapy IgA/C3. Conclusion: These findings suggest that the serum IgA/C3 ratio may be a useful marker to predict disease activity and histologic severity in HSN.